Project Summary Small cell lung cancer (SCLC) is responsible for over 30,000 deaths each year in the United States alone. SCLC has a two-year survival rate of ~6% and unlike the other major subtypes of lung cancer, there are currently no targeted therapies approved for SCLC. SCLC is initially highly responsive to chemotherapy, but rapidly develops resistance leading to mortality in ~10 months. Clearly, a major unmet need for the treatment of SCLC is the identification of new therapeutic targets and treatment strategies to combat chemo-resistant disease. Our understanding of chemotherapy resistance mechanisms has been hampered by a lack of relapsed human SCLC tissue due to rare surgical resections. In addition, there have been few mouse models of the disease that exhibit short latencies and chemo-sensitivity. To address these challenges, we performed whole exome sequencing on relapsed SCLC from 30 patients. Relapse-specific genomic alterations in the WNT/APC/?-catenin pathway were identified in ~66% of relapsed SCLC suggesting that this pathway promotes chemo-resistance. Second, we developed a novel mouse model of SCLC driven by loss of Rb1, Trp53 and overexpression of Myc?three of the most common genetic alterations in the human disease. Mice develop SCLC within weeks that highly resembles the human disease at the level of histopathology, biomarker expression and chemo-sensitivity followed by relapse. This model will be a useful tool to test candidate chemotherapy resistance mechanisms and identify novel therapeutic targets that inhibit chemo-resistance. The objective of this study is to use this novel mouse model and comprehensive genomic analyses of primary and relapsed human SCLC to identify mechanisms of chemotherapy resistance and novel therapeutic targets to combat chemo-resistant disease. We hypothesize that activation of the WNT/?-catenin pathway promotes chemo-resistance in SCLC and that targeted inhibition of the pathway will inhibit chemo-resistant disease. We predict that expansion of our genomic and transcriptomic profiling will identify additional novel pathways involved in chemo-resistance. To test these hypotheses, we will: 1) identify recurrent pathway alterations in relapsed human and mouse SCLC using whole genome, exome, transcriptome and epigenome sequencing and 2) functionally determine whether canonical WNT/?-catenin signaling and other candidate pathways are necessary and sufficient for chemo-resistance in SCLC in vivo. This approach is innovative because we will employ unbiased comprehensive genomic and epigenomic analyses on relapsed human SCLC and a novel immune-competent mouse model of SCLC that recapitulates key features of the human disease. The WNT/?- catenin pathway is largely unexplored in SCLC. This research is significant because there are currently no targeted therapies approved for SCLC. A better understanding of the critical pathways driving chemo- resistance in SCLC will impact the treatment and survival of patients with this intractable disease.